The introduction of polyethylene as jacketing material about telephone wires and cables was an important development in the field of telephonic communications. Polyethylene was found to be corrosion-resistant, tough, abrasion-resistant and served to adequately protect the inner core of telephone wires and cables. Although polyethylene has properties which makes it desirable for use as jacketing material, as described, it also has other properties which presented problems. For example, polyethylene was found to degrade upon aging. Consequently, antioxidants were added to polyethylene in order to increase its working life. It was also found that polyethylene was degraded by the ultra-violet rays of the sun. This problem was solved by adding carbon black to the polyethylene.
One problem which still causes concern to the communications industry is the environmental stress cracking of polyethylene wires and cables. Communication wires and cables, obviously, have to be pulled through conduits as communication systems are being installed. In order to facilitate movement of the wires and cables through these conduits, it is customary to lubricate the polyethylene jacketing with surface active agents such as soaps and detergents. As the lubricated polyethylene wires and cables are stressed, on being pulled through the conduits, the lubricants induce large, full depth cracks to form in the polyethylene jacketing causing catastrophic failure of the wires or cables.
Catastrophic failures of polyethylene wire and cable due to environmental stress cracking has led to the adoption of stringent tests by the communications industry with respect to compositions proposed for use as jacketing material. These tests, which are two in number and are subsequently described, are referred to as United States industry standards for stress cracking resistance. As a rule, failure of either test renders a composition unacceptable, to domestic users, for wide use as jacketing material about telephone wires and cables.
The present invention provides compositions which pass both tests and consequently are particularly useful as material which can be extruded about telephone wires and cables, providing jacketing therefor. The compositions of this invention are especially attractive, from a commercial standpoint, as they are characterized by improved stress cracking resistance with no significant change in other physical properties such as yield strength, elongation and tensile strength.
The compositions of this invention comprise a mixture of a branched, low density, high molecular weight polyethylene having a melt index of 0.08 to 0.25 and a density of 0.919 to 0.923, and a normally solid copolymer of ethylene and an alkyl acrylate, having a melt index of about 3 to about 24, wherein the combined alkyl acrylate content is about 16 to about 25 percent by weight. The copolymer is present in the compositions in an amount which passes United States industry standards for stress cracking resistance, generally in an amount of about 6 to about 25 percent by weight, preferably in an amount of about 10 to about 20 percent by weight based on the combined weight of the polyethylene and the copolymer of ethylene-alkyl acrylate. The actual amount used will vary and depend upon the melt index of the copolymer as well as the combined alkyl acrylate content thereof.
As to the tests conducted, referred to as United States industry standards for stress cracking resistance, they were conducted as follows: